The present invention relates to improvements in devices which regulate the movement of control rods within the closed vessel of a nuclear reactor.
It is known that the reactivity of a nuclear reactor is generally regulated by regulating the insertion, within the core of the same, of control rods which have the property of absorbing neutrons, and slow variation in the level of insertion of these rods makes it possible to control the power level supplied by the core, and complete and rapid insertion of these makes it possible to shut down the reactor rapidly.
These slow movements of the rods are generally carried out using devices employing male and female threaded members, the nut member being locked in rotation and the screw member being locked in its translatory movement. Such devices are, for example, described in French Pat. Nos. 1,241,783 and 1,450,728 and include, among other provisions, a mechanism which makes it possible to carry out rapid insertion of the control rod by disengaging the screw/nut assembly, with the nut being left continuously in engagement with the screw.
These known devices use a device such as a grab or a hook for supporting, when in the closed position, the screw/nut assembly, and this is actuated by an electromagnet the coil of which is located under the stator of the motor provided for rotating the screw portion of the said screw/nut assembly. Rapid insertion of the control rod into the core of the reactor is obtained by shutting off the electrical supply to the coil of the electromagnet, which leads to opening of the grab arrangement, freeing of the screw/nut assembly, and, as a result of this, release of the control rod which falls into the core of the reactor until it reaches its position of maximum insertion.
This known arrangement, however, suffers from one disadvantage. This consists in the fact that safetly rules applicable to nuclear reactors lay down that in case of earthquake it must be, firstly, possible to carry out rapid insertion of the control rod and, secondly, that the vessels providing for retention of the primary fluid remain intact. However, the presence of supplementary pole pieces and windings, located under the stator of the motor for driving the screw, causes the mechanism to be greater in length and heavier than desirable, which has the effect of increasing the stresses and deformations to which this mechanism might be subject under earthquake conditions.